Provide the ability to register REST event handler at certain points during the cluster life-cycle

There should the ability to register an REST event handler at specific points during the cluster provisioning life-cycle. The following life-cycle events are requested:
- Hadoop cluster provisioning request
- VMs are booted
- Hadoop cluster is operational
- Hadoop cluster termination (including cancellation of cluster provisioning)
- Cluster is set to Error state
- Scaling Cluster (add/remove of node)

It should be possible to propagate the tenant ID and cluster ID in the URI invocation. This feature could be exposed as a set of properties on the cluster object/template and should be made optional.

Use Cases for above include the ability to:
- Provision user accounts once VMs are booted
- Create specific HDFS directories once cluster is operational
- Send notification to tenant/user
- Start data ingest process